1. Field of the Invention
The present invention relates to a muffle tube inspection method for detecting a crack generated at a muffle tube constituting a sintering furnace performing dehydration and sintering when manufacturing a silica glass-based optical fiber preform, and a manufacturing method of a silica glass-based optical fiber preform including an inspection step using the inspection method.
2. Description of the Related Art
Generally, a silica glass-based optical fiber is manufactured in a manner such that a porous preform produced by a VAD (Vapor phase Axial Deposition) method or an OVD (Outside Vapor Deposition) method is subjected to a high temperature heat-treatment, a transparent glass body is thereby formed, and the transparent glass body is fiber-drawn.
A step of forming a transparent glass body so that the porous preform is subjected to a high temperature heat-treatment is a dehydration and sintering step.
Conventionally, the dehydration and sintering step is carried out using a sintering furnace 100 as shown in FIG. 3.
The sintering furnace 100 is substantially configured to include a removable lid 101, a muffle tube 102 made of silica glass, a heater 103 placed around the periphery of the muffle tube 102, and a furnace body 104 covering the heater 103 and shutting external air out.
In the dehydration and sintering step, it is necessary to heat up the inside of the muffle tube 102 to approximately 1500° C. to form the porous preform to be a transparent glass, however, the muffle tube 102 becomes soft at the temperature.
Consequently, when there is significant difference between the inner pressure and the external pressure of the muffle tube 102, the muffle tube 102 sometimes deforms.
For this reason, in order to measure differential pressure between inner pressures of the muffle tube 102 and the furnace body 104 and atmospheric pressure (external pressure), an inner-pressure gauge 105 measuring the inner pressure thereof is provided at the muffle tube 102, and inner-pressure gauge 106 measuring the inner pressure is provided at the furnace body 104.
Additionally, a balloon shaped pressure-adjustment device 107 for adjusting the inner pressure of the muffle tube 102 based on the measuring results of the above-described differential pressure is provided at the upper portion of the muffle tube 102 (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2000-169173 and Japanese Unexamined Patent Application, First Publication No. 2005-194173).
Additionally, a weight 108 pressing the pressure-adjustment device 107 by the weight thereof to adjust the inner pressure of the muffle tube 102 is mounted on the balloon-shaped pressure-adjustment device 107.
In the dehydration and sintering step using the sintering furnace 100, a porous preform 201 supported by a support rod 202 is inserted into the inside of the muffle tube 102 from above thereof.
Thereafter, in order to dehydrate the porous preform 201 or remove impurities, helium, halogen series gases, or the like is introduced into the muffle tube 102 through a gas supply port 109 that is connected to a bottom portion of the muffle tube 102 and communicated with the inside thereof, and the inside of the muffle tube 102 becomes a mixed gas atmosphere including helium, halogen series gases, or the like.
Subsequently, the heater 103 heats up the porous preform 201 from the bottom portion thereof toward the upper portion thereof by moving downward the porous preform 201 to a lower side of the muffle tube 102 while rotating the support rod 202 as the central axis.
Accordingly, the dehydration of the porous preform 201 is carried out from the bottom portion thereof toward the upper portion thereof, and a transparent glass is formed.
Furthermore, an inert gas such as helium gas, argon gas, or the like is introduced into the furnace body 104 through a gas supply port 110 that is provided at a side face of the furnace body 104 and communicated with the inside thereof.
Next, the inert gas that was introduced into the furnace body 104 is discharged to an exterior through a gas exhaust 111 that is provided a side face of the furnace body 104 and communicated with the inside thereof.
As stated above, the inside of the furnace body 104 is in an inert gas atmosphere by introducing the gas into the furnace body 104 and discharging the gas.
However, when a cracked muffle tube 102 is used in the dehydration and sintering step, a dehydration defect of the porous preform 201 may occur, or impurities generated from the heater 103 or the like may be introduced into the porous preform 201.
Because of this, transmission characteristics of the optical fiber manufactured by fiber drawing from the resultant transparent glass body becomes deteriorated.
Additionally, in the case of using a halogen series gas such as chlorine, fluorine, or the like, if the gas leaks outside the muffle tube 102, safety or workability loses.
For this reason, a sintering furnace in which a gas detecting sensor detecting helium, halogen series gases, or the like is provided at the furnace body is proposed in order to detect a crack generated at a muffle tube in the dehydration and sintering step (for example, refer to Japanese Unexamined Patent Application, First Publication No. H5-78140, Japanese Unexamined Patent Application, First Publication No. H11-1337, and Japanese Unexamined Patent Application, First Publication No. 2000-226224).
Such sintering furnace can detect a crack generated at a muffle tube when the gas to be detected flows in the muffle tube, particularly, only when dehydration and sintering are carried out.
However, there is a concern that muffle tube is cracked not only when the dehydration and sintering is carried out but also when the muffle tube is cooled down after completion of sintering the porous preform, or due to an external factor such as an earthquake or the like.
Particularly, in the muffle tube, the portion surrounded by the heater, at which difference in temperature between the case of performing dehydration and sintering and the case of not performing dehydration and sintering is large, is easily cracked.
If the dehydration and sintering step begins without being aware of a cracked muffle tube, there are problems in that, as above-described, a dehydration defect in the porous preform occurs in the dehydration and sintering step or impurities generated from the heater or the like is introduced into the porous preform.